Biosafe removal of diclofenac from wastewaters by a continuous-flow mode cold atmospheric pressure plasma system

Diclofenac (DCF) is not only a versatile non-steroidal anti-inflammatory drug, but also an emerging water pollutant. There is an urgent need for innovative technology of wastewater treatment to reduce the impact of DCF on the environment. Thus, we examined the applicability of pulse-modulated radio-frequency atmospheric pressure glow discharge (pm-rf-APGD) for degradation of 1.56-50.0 mg L-1 DCF from both aqueous solutions and synthetic wastewaters. The removal rates varied from 84.0 +/- 3.8% to 100.0 +/- 0.1% for the aqueous solution and increased together with the decreasing concentration of DCF. For synthetic wastewaters, the DCF removal was enclosed in the range from 21.0 +/- 1.0% to 74.8 +/- 0.1%. The energy efficiencies varied from 0.0116 to 0.312 g kW-1h-1 for aqueous solutions and from 0.00779 to 0.0087 g kW-1h-1 for synthetic wastewaters, respectively. By application of ultraperformance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS) and gas chromatography-mass spectrometry (GC-MS), the pm-rf-APGD-driven DCF degradation pathways were revealed to involve decomposition, nitration, and dehalogenation reactions yielding dichloroaniline, nitrodiclofenac and 8-chlorocarbazole-1-acetic acid, respectively, before achieving the total decomposition of DCF to carbon dioxide, water, chlorides, nitrates and amines. Notable increases in the contents of the radicals, i.e., H2O2 in addition to NO2- and NO3- , were observed in the plasma-treated DCF solutions in contrast to the controls. Finally, no cytotoxic activity of the pm-rf-APGD-treated DCF solutions was demonstrated on model nonmalignant human endothelial cells (HUVEC) and human immortalized keratinocytes (HaCaT). We anticipate that the described continuous-flow pm-rf-APGD-based reaction discharge system might be applied for biosafe removal of DCF from the effluents originating from wastewater treatment plants.